150 litres BR tuned to 35 Hz (EBS) gets you down to about 33Hz - plenty of lows for PA in a fairly compact box! Not the most efficient box on the Earth, though (ca 94 dB/W/m at 40 Hz), but still compact (700x700x420 outer dimentions when 18 mm plywood used). With 3kW you get about 128 dB at 40 Hz. A good 2x18" BR can get 130 dB with only 1 kW. But EBP of 107 would be fairly good for horns too, I quess.